As magnetic read/write heads have been required to deal with magnetic media having increasingly higher area density of recorded information, various methods have been developed to improve the capabilities of the head to read and write at those levels. Traditionally, the direction taken in trying to achieve the reading and writing of this high density information has been to decrease the spacing (i.e. the static fly height) between the disk and the slider in each new generation of products.
FIG. 1 is a schematic illustration showing a single suspension-mounted slider (the combination collectively termed a “head gimbals assembly (HGA)”) positioned above a spindle-mounted, rapidly rotating magnetic hard disk during disk-drive operation in a hard disk drive, HDD, (or a spin-stand) at ambient operating temperature. The suspension (101) holds the slider (10) at an angle above the surface of the spindle-mounted magnetic disk (400), producing a “fly height” (clearance) between the air bearing surface (ABS) (100) of the slider and the disk. A read/write head (600) is mounted within the slider. The rotation of the disk (400) is, by definition, into the leading edge of the slider, while the read/write head (600) is located at the trailing edge of the slider. The write-gap (30) (which the write magnetic field spans and contacts the disk) of the head (90) is “above” (i.e. to the trailing edge side of) the read-gap portion (30). The hydrodynamics of the air layer between the ABS and the rotating disk surface supports the slider at its fly height above the disk. In a dynamic flying height (DFH) type of system to be considered herein, a controllable heater element (35), is located adjacent to the write gap (90) and, by heating the region surrounding the gap, can cause protrusions (not shown) of the ABS (200) of the head portion relative to the undisturbed shape of the ABS when it is not heated. These protrusions will produce a characteristic shape (the protrusion profile) across the ABS, which will manifest itself in variations of the flying height of the ABS above the disk. It is to be noted that modern HDD systems contain multiple disks and multiple read/write heads that are aligned with each of the multiple disks. Therefore, the method to be discussed herein may be discussed in terms of a single disk and its head, but the method is in no way limited to a single head/disk combination and it may, by extension, be applied to a multiplicity of disks and their associated heads. Similarly, the steps required to implement the method as discussed herein are easily implemented in a single head/disk combination or, independently, in a multiple head/disk combination. Finally, all steps required to implement the method can be implemented in hardware or firmware incorporated within the HDD, the spin stand or the multiple disk HDD.
Consistent and rapid increase in the recording area density of hard disk drives requires a corresponding continuous decrease in the flying height of the slider or mechanical spacing between magnetic recording head and disk. After the FH was reduced to about 10 nm, further decrease in FH became extremely difficult. Now that the thermal expansion based technique of DFH has emerged, dynamic control of the flying height during disk rotation has become possible. This technology has been widely applied in the past several years. As the recording density approaches 150 Gbit/cm2 (1 Tbit/in2), the spacing must now be decreased to a range of 1 nm.
In order to reliably control the spacing through activation of the heater, it is necessary to have a feasible way of measuring the spacing while applying the power to the heater. Relative spacing change can be calculated based on the well-known Wallace equation that relates signal loss to spacing as a function of frequency. However, to determine the actual spacing, a reference point is needed. The reference point is usually taken as the point where the head touches the disk. It is defined as the zero of the spacing. The process of finding this reference point is called touch down (TD) detection. For better TD detection and potential real time monitoring of head/disk spacing, the head element typically also includes a head-disk interference (HDI) sensor (or, HDIs). This sensor is a resistive temperature sensor used to detect a temperature change in the head that is induced by changes in clearance during head vibrations or by a direct contact caused by contacting with disk asperities. Note that different sensor types exist, including PZT and LDV sensors. The HDIs signal (from whatever type sensor being utilized) has DC (low frequency) and AC (high frequency) components. When the slider flies at a low clearance, low frequency oscillation (the DC component) appears. When the slider contacts the disk and afterwards, a strong high frequency (the AC component) HDIs signal appears. Thus, the AC component of the HDIs signal is more sensitive to the slider/disk contact, and, therefore, it could be more effective for TD detection. After a reference point is found, a desired spacing can be set to a specified value, such as 1.5 nm for the current generation of drives, by adjusting the DFH power during reading and writing.
In the current generation of drives, where the whole disk is divided into 10 or 30 zones in a radial direction, the TD power and spacing at each zone should be a constant. In fact, the TD power and the spacing are not even constant along the same track. The spacing fluctuates because the disk within the disk drive does not present a perfectly flat surface. For example, the disk typically has an initial distortion from disk manufacturing. After the disk is assembled into the drive, additional distortions or imperfections can be induced. At an inner diameter (ID) region, due to forces applied in clamping the disk, the disk might have a large local distortion. At an outer diameter (OD) region, due to a poor alignment, the disk might have a tilting relative to the disk rotational axis. Both the force-induced distortion of the disk and its tilting due to misalignment will induce a spacing fluctuation at the same track. The fluctuation amplitude ([max spacing-min spacing], during one revolution at the same track) is dependent on the disk condition (distortion/tilting) and the slider air bearing surface design. The amplitude could range between 0.5 nm and 2 nm. When the spacing approaches the 1 nm range, this fluctuation becomes very significant. At the minimum spacing position, the slider might actually contact the disk, which causes a system failure if the contact occurs during a writing process. At the maximum spacing position, the total spacing is too large, and it can cause a “weak write” failure due to a large magnetic spacing and a magnetic field that is insufficient at the disk to create a proper magnetic transition. For these reasons, the spacing fluctuation needs to be under control or compensated which, in turn, requires an accurate method of measuring the spacing and its fluctuations.